Rotary face seals typically include a stator or sealing ring fixed in place and a rotor or mating ring which rotates with a rotating machine part such as a shaft. The sealing faces of both the sealing ring and the mating ring are very close together but not touching. A pressurized fluid film (e.g., air) is formed between the sealing faces to separate them and prevent wear due to friction.
The fluid film is typically formed by a set of spiral grooves cut into the sealing face of either the mating ring or the sealing ring. When the mating ring rotates, the fluid is forced (pumped) from the outer diameter of the two rings inward to the inner diameter (or vice versa) with sufficient pressure to separate the sealing faces and form the sealing layer. See U.S. Pat. No. 5,769,604 incorporated herein by this reference.
Non-contacting, film-riding face seals have been used for industrial applications successfully ever since they were first introduced in 1969. Face seals are characterized by extremely low leakage and low wear. Because of these features, there has been a continuous effort in the aerospace industry to develop non-contacting face seals for large diameter gas turbine engines.
There are, however, two major difficulties associated with using face seals for high rotational speed and large shaft diameter turbomachines. First, it is difficult to control the flatness of the seal faces because of their size. Second, the seal faces of both the rotor and stator can cone either inward or outward due to large thermal and pressure effects. A negative deflection causing a divergent flow path can be disastrous for standard hydrodynamic face seals since the flow of gas into the region between the faces is then cut off. With standard hydrodynamic face seals, the deflection is expected to be much larger than the film thickness than the face seal runs on. Large positive coning can also result in failure for large diameter face seals because the resulting weak film stiffness increases the chance of face contact.
Realizing that conventional spiral groove face seals will not survive the harsh environment encountered in large turbine engines, the '604 patent proposes a spiral groove design which initially showed some promise in large turbine engine applications. Two sets of seal sections, two feed grooves, and an additional dam section are used for the face seal. This configuration increases the film stiffness about three times compared to a conventional spiral groove face seal design providing the stator with more power to adapt to the deflection of the rotor. The major limitation with the design set forth in the '604 patent, however, is that it always has a seal section pumping from an edge (the outer diameter) to the center thus limiting the ability to prevent edge contact. In addition, the seal pumping groove configuration disclosed in the '604 patent is fairly complicated.